1. Field of the Invention
The present invention relates to an improvement of a non-feedback type load current controller for controlling the current supply rate (i.e., duty factor) corresponding to the rate between a current supply term and a current supply period of a semiconductor opening/closing element for driving a current consumer in response to the values of supply target current and driving power source voltage for an inductive electric load such as an electromagnetic solenoid for driving a shift gear switching electromagnetic valve in a multistage automatic transmission for a vehicle, for example.
2. Description of the Related Art
A control circuit for driving an electrical load has been generally and broadly used to detect occurrence of breaking of wire/short-circuiting of an electrical load or an opening/closing element or detect breaking of load wires or occurrence of abnormality of high-level short-circuit (i.e., short-circuit to the positive side of the power source)/grounding and notify abnormality, or prevent burnout of the opening/closing element.
For example, JP-A-8-293414 discloses the following solenoid failure detecting device. That is, in a current supply circuit including a current detecting resistor, an opening/closing element and a solenoid as an inductive electrical load which are connected to one another in this order from a driving power source, the voltage between both the ends of electrical load is converted to binary values of “H” and “L”, and input to an operation unit as a microprocessor. The presence or absence of wire breaking of the electrical load is detected when the opening/closing element is opened (i.e., under an open circuit condition), and the presence or absence of short-circuiting of the electrical load is detected when the opening/closing element is closed (i.e., under a close circuit condition). When the open circuit condition of the opening/closing element is normally continued, the opening/closing element is closed (i.e., set to a close circuit condition) during only a short time period when the electrical load does not operate effectively to thereby detect the presence or absence of short-circuiting. When the close circuit condition of the opening/closing element is normally continued, the opening/closing element sets the open circuit condition during only a short time period when the electrical load is not non-operated to thereby detecting the presence or absence of breaking.
Furthermore, JP-A-5-217737 discloses a linear solenoid driving device including a current detecting unit for detecting current flowing in a linear solenoid, an error signal generating unit for integrating the difference between the detection result of the current detecting unit and target current to generate an error signal, a current supply control unit for controlling the current supply to the linear solenoid on the basis of the error signal generated in the error signal generating unit, and a failure detecting unit for comparing the error signal generated in the error signal generating unit with a preset reference signal in magnitude and detecting a failure of the driving device in accordance with the comparison result.
The driving device of the linear solenoid is constructed as a current supply circuit by connecting an opening/closing element as the current supply control unit, a linear solenoid as the electrical load and a current detecting resistor as the current detecting unit in series in this order and then connecting these elements to a driving power source.
The solenoid failure detecting device disclosed in the JP-A-8-293414 has no current control function for the solenoid, and it is merely designed so that the power supply state and the power non-supply state are switched to each other by the opening/closing element. Accordingly, for example, in the case of occurrence of a semi-breaking state where the power supply circuit resistance is slightly increased due to incomplete contact of a connection connector containing in the power supply circuit or in the case of occurrence of a semi-short-circuiting state where the load resistance is slightly reduced due to interlayer short-circuiting of a coil of the solenoid or the like, the solenoid failure detecting device described above cannot detect such an abnormality sign as described above.
Furthermore, subtle time control is needed to inversely operate the opening/closing element for only a short time without affecting the operation state of the electrical load and check abnormality in the close circuit period or open circuit period of the opening/closing element, and thus there is a risk that the electrical load erroneously operates in an unintentional state.
Furthermore, the linear solenoid driving device disclosed in JP-A-5-217737 is designed so as to carry out feedback type current control based on current detection. Even when the semi-breaking state or the semi-short-circuiting state as described above occurs, the power supply voltage is automatically corrected so that predetermined target current can be achieved, whereby the error signal is prevented from increasing.
Accordingly, if the resistance of the electric load does not vary over a level at which the correction based on the feedback control is difficult, an abnormality state cannot be detected, and thus any abnormality sign cannot be detected.
Furthermore, when the connection point between the opening/closing element and the electrical load is grounded to a ground circuit due to mixed touch, the stress applied to the opening/closing element would be excessively increased even if the opening/closing element is urgently interrupted because the current detecting resistor does not function as a current limiting resistor for the opening/closing element. Therefore, there is a risk that the opening/closing element is damaged.